Mechanical part made of steel having high properties and process for manufacturing same

ABSTRACT

A mechanical steel part in steel with high characteristics, characterized in that its composition, comprising in weight percentages, is 0.05%≦C≦0.25%; 1.2%≦Mn≦2%; 1%≦Cr≦2.5%; wherein the contents of C, Mn and Cr are such that (830-270C %-90 Mn %-70Cr %)≦560; 0&lt;Si≦1.55; 0&lt;Ni≦1%; 0&lt;Mo≦0.5%; 0&lt;Cu≦1%; 0&lt;V≦0.3%; 0&lt;Al≦0.1%; 0&lt;B≦0.005%; 0&lt;Ti≦0.03%; 0&lt;Nb≦0.06%; 0&lt;S≦0.1%; 0&lt;Ca≦0.006%; 0&lt;Te≦0.03%; 0&lt;Se≦0.05%; 0&lt;Bi≦0.05%; 0&lt;Pb≦0.1%; the remainder of the steel part being iron and impurities resulting from processing, and wherein the in that its structure of the steel is bainitic and contains no more than a total of 20% of martensite and/or pro-eutectoid ferrite and/or pearlite.

The invention concerns steels for mechanical parts having high characteristics obtained by hot forging or bar machining.

Some steel grades allow the obtaining of high mechanical characteristics for forged parts or as-rolled bars without the use of controlled cooling or subsequent heat treatment. They are based on the obtaining of a homogeneous bainitic microstructure.

Said grades have already been proposed such as those the subject of documents EP-B1-0 787 812 or EP-A-1 426 453, which are given industrial use for the production of forged parts for internal combustion engines. However, to obtain high mechanical characteristics on the parts described in these documents it is necessary, unless being restricted to diameters of the order of 20 mm, to use carbon contents of 0.25% or higher.

While it is currently possible to guarantee a tensile strength of the order of 1200 MPa after natural cooling, in particular when using grades such as those described in EP-A-1 426 453, the obtaining of these mechanical characteristics is often achieved at the expense of an impact strength of 30 J·cm⁻² or lower.

It is the objective of the invention to propose mechanical parts made of a determined steel grade which, combined with adequate heat and thermo-mechanical treatments, allow the simultaneous obtaining of advantageous mechanical properties (tensile strength Rm, yield strength Re, Re/Rm ratio, elongation A, percentage reduction of area Z) and improved impact strength KCU compared with known mechanical parts in steel for this same use.

For this purpose the subject of the invention is a mechanical part having high characteristics characterized in that its weight percentage composition is as follows:

0.05%≦C≦0.25%;

1.2%≦Mn≦2%;

1%≦Cr≦2.5%;

(830-270C %-90Mn %-70Cr %)≦560;

traces≦Si≦1.5%;

traces≦Ni≦1%;

traces≦Mo≦0.5%;

traces≦Cu≦1%;

traces≦V≦0.3%;

traces≦AI≦0.1%;

traces≦B≦0.005%;

traces≦Ti≦0.03%;

traces≦Nb≦0.06%;

traces≦S≦0.1%;

traces≦Ca≦0.006%;

traces≦Te≦0.03%;

traces≦Se≦0.05%;

traces≦Bi≦0.05%;

traces≦Pb≦0.1%;

the remainder being iron and impurities resulting from processing, and in that the structure of the steel is bainitic and contains no more than a total of 20% martensite and/or pro-eutectoid ferrite and/or pearlite.

Preferably, traces≦Si≦0.3%.

Preferably, 0.8≦Si≦1.5%.

Preferably, Ni≦0.5%.

Preferably, 0.04%≦Mo≦0.5%.

Preferably, 0.5%≦V≦0.3%.

Preferably, 0.005%≦Al≦0.1%.

Preferably, 0.0005%≦B≦0.005%, and traces≦N≦0.0080% and Ti %≦3.5N %.

Preferably, 0.005%≦Ti≦0.03%.

Preferably, 0.005%≦S≦0.1%.

A further subject of the invention is a process for manufacturing a part in steel such as a mechanical part having high characteristics, characterized in that it consists of the following steps:

-   -   a steel blank or bar is prepared whose composition conforms to         the above;     -   the blank or bar is hot worked in the austenitic region by         forging or rolling;     -   the hot-worked blank or bar is cooled at a rate imparting a         bainitic structure thereto containing a total of no more than         20% martensite and/or pearlite and/or pro-eutectoid ferrite;     -   and optionally one or more machining operations to impart the         final dimensions and surface condition to the part.

Before or after the machining(s), tempering can be carried out at between 200 and 350° C. for 30 minutes to 4 hours.

The hot-worked blank or bar can be cooled naturally in still air.

The hot-worked blank or bar can be cooled under forced air.

As will have been appreciated, the invention is based on a composition of the part and its combination with a metallurgical structure that is 80% bainitic or more, able to be obtained using simple means such as cooling in still or forced air.

By <<bainite>> is meant both pure bainite and the bainite+residual austenite mixture if austenite is present as is frequently the case, and all possible bainite morphologies are included including intragranular bainite (also called acicular ferrite). The other phases which may be present, namely martensite, pro-eutectoid ferrite and pearlite, must not represent more than 20% of the structure.

The grades used in the present invention, by having recourse to so-called <<low-medium>> carbon and by lowering the point of transformation onset chiefly through the incorporation of chromium and manganese, allow the obtaining of tensile strengths in the region of 1200 MPa or higher, with impact strengths of at least 40 J·cm⁻², possibly even reaching 70 J^(SM)cm⁻².

However, these grades in the hot forged or as-rolled state exhibit Re/Rm ratios of the order of 0.6 and hence significantly lower yield strengths than those obtained with quenched-tempered grades having the same mechanical strength.

Yet as will be shown, it is also possible according to the invention, using subsequent low temperature tempering, to increase the yield strength most significantly (by approximately 25%) without however increasing mechanical strength. This type of tempering is to be distinguished from the tempering sometimes used on micro-alloy steels conducted in the region of 550-650° C. which allow the precipitation of alloy carbides. Whereas this type of tempering is often accompanied by a major loss of impact strength, tempering at low temperature as performed in the invention has a beneficial effect on impact strength (being increased by up to about 30%).

Support for the choice of composition ranges will now be given for the various elements of the grade of the parts according to the invention. All contents are given in weight percentages.

The C content is between 0.05 and 0.25%. This range called the <<low-medium carbon>> range since its upper limit lies in the lower region of the contents considered to be medium carbons and its lower limit belongs to the region of low carbons, provides for a very homogenous microstructure and hardness even in the presence of segregations. In particular, for carbon contents of less than 0.2% the hardness of martensitic microstructures optionally present in the segregated regions is only slightly higher than that of the bainitic microstructure. In addition, these carbon contents allow ductility and impact strengths to be obtained that are higher than those obtained, for the same level of mechanical strength, with contents higher than 0.25%.

The Mn content is between 1.2 and 2%. Manganese is used jointly with chromium as main element to lower the bainite formation onset temperature (Bs) during continuous cooling. Since a relatively low carbon content is used, relatively high contents of Mn are required which additionally must contribute to meeting the condition regarding C, Mn, Cr contents for calculating Bs (see below). Manganese is limited to 2% to avoid excessive segregation problems.

The Cr content is between 1.2 and 2.5%. In the present invention, Cr is used for the same reason as Mn to lower the onset temperature of bainitic transformation Bs.

The contents of C, Mn and Cr must additionally be such that 830-270 C %-90Mn %-70Cr %≦560.

The onset temperature Bs of bainitic transformation can be conventionally estimated using the following formula:

Bs=830-270C %-90Mn %-70Cr %-37N %-83Mo %

where the contents are expressed in weight percentages (see for example Bhadeshia, Bainite in Steels, IOM 2001). In the invention, having regard to the relatively low contents of Ni and Mo in the steel, it is possible only to give consideration to the contributions made by C, Mn and Cr. At all events, if Ni and Mo are present in the upper content ranges examined below they will contribute towards lowering Bs. It is therefore ensured that in any case a Bs will be obtained of 560° C. or lower.

Si is contained between traces and 1.5%. Silicon can be used to avoid the formation of carbides which would deteriorate impact strength during bainitic transformation. With carbon contents of less than 0.2% however this formation of carbides remains small and the addition of Si loses its advantage from this viewpoint. Also, by promoting the formation of residual austenite, Si allows an improvement in the fatigue resistance for some applications. In some cases, however, its use may also be excluded through the need to avoid excessive surface decarburization. Two variants of the invention can therefore be envisaged. In a first variant the Si content merely results from the processing conditions for the bath of liquid metal, namely from the raw materials used and possible partial oxidation of Si, and no deliberate major addition of Si is made. In this case the Si content obtained typically ranges from traces to 0.3%. In a second variant, Si is purposely added, preferably to obtain a content of 0.8 to 1.5%.

Ni is contained between traces and 1%, preferably between traces and 0.5%. It may solely be present through its addition via the raw materials as residual element, or it can be added in a small amount to contribute towards reducing Bs temperature. However its content is limited to 1%, better still 0.5% for cost-related reasons, this element being costly and its price is likely to fluctuate heavily on the market.

Mo is contained between traces and 0.5%, preferably between 0.04 and 0.5%. The role of molybdenum on quenchabiity has been well established: it allows the formation of ferrite and pearlite to be avoided but does not however slow the formation of bainite. It can therefore be added in variable quantity depending on the diameter of the part. A second advantage of molybdenum is that it limits the sensitivity to reversible temper brittleness (see Bhadeshia, Mater. Sci. Forum, High Performance Bainitic Steels, vol. 500-501, 2005). Finally, molybdenum strengthens austenite by changing to a solid solution. Insofar as the mechanical strength of austenite is one of the main factors governing the fineness of a bainitic structure (Singh and Bhadeshia, Mater. Sci. Eng. A, 1998, Vol 245, p. 72), the addition of Mo indirectly contributes to obtaining a finer structure. The upper limit is mainly fixed for economic reasons.

V ranges from traces to 0.3%, preferably between 0.05 and 0.3%. The addition of vanadium allows additional hardening; however contrary to the case with ferrite-pearlite steels, this hardening does not appear to occur via precipitation; it has effectively been experimentally shown that after hot working (hot forging or rolling) and natural cooling, only a very small fraction of vanadium is in precipitated form. Just like molybdenum, vanadium reinforces austenite by precipitation and solid solution, and it can therefore indirectly contribute towards the fineness of the bainitic structure, hence its hardening effect. The addition thereof is limited to 0.3% for economic reasons.

The Cu content ranges from traces to 1%. It may optionally be used to contribute towards hardening, but would lead to implementation difficulties with contents higher than 1%.

The Al content ranges from traces to 0.1%, preferably between 0.005 and 0.1%. Al is optionally added to ensure deoxidation of the steel and to avoid the excessive growth of austenitic grains when held at high temperature (e.g. for case hardening) which may be performed subsequently on the part after the implementation of the process of the invention.

B ranges from traces to 0.005%, preferably between 0.0005 and 0.005%. This optional element can be used for parts of large diameter, in particular if the Mo content is low, to guarantee the homogeneity of the structure (limit the presence of ferrite). In this case, it is preferable to combine the addition of B with an addition of Ti which will capture the nitrogen to form nitrides and thereby avoid the formation of boron nitrides. In this manner, all the boron will be available to play its role of homogenizer of the structure. This should give traces≦N≦0.0080% and Ti %≦3.5N %.

Ti ranges from traces to 0.03%, preferably between 0.005 and 0.03%. As just mentioned, this optional element is chiefly to be used for grades with boron, with the ratio of Ti % to N % just indicated.

Nb is contained between traces and 0.06%. This optional element can be used to refine the austenitic structure after hot forging or rolling, resulting in a reduction in the sizes of bainite packets and accelerated bainitic transformation (Bhadeshia, Proc. Royal Soc., 2010, Vol 466, p 3).

S is contained between traces and 0.1%. As is well known this element may optionally be left at a relatively high level, even purposely added to improve the machinability of the steel. Its content is then 0.005 to 0.1%. Preferably this significant presence of S is accompanied by an addition of Ca of up to 0.006%, and/or Te of up to 0.03%, and/or Se of up to 0.05%, and/or Bi of up to 0.05% and/or Pb of up to 0.1%. This improved machinability can particularly be sought for applications in which the part is subjected to fatigue stress or for applications in which its mechanical properties are improved at least locally by sufficient pre-stressing to prevent the propagation of cracks (roller burnishing of crankshafts, autofrettage of high pressure injection rails).

The other elements contained in the steel of the invention are iron and impurities resulting from processing, present in their usual contents.

The preferred ranges cited for the various elements are independent of each other. A steel that comes under only one or some of these preferred ranges and not under others would therefore be considered as being included in the invention.

Industrially, the part can be produced by hot working a blank or bar having the above-described composition, such as hot forging or hot rolling, or by machining a ready-to-use bar.

In the first case the industrial process entails a hot working step conducted in austenitic phase (typically 1100-1250° C.) followed by natural cooling. One of the important points of the invention is the possible obtaining of high mechanical characteristics without the use of heat treatments after forging or rolling, or any particular highly restricting control procedure over the rate of cooling which can take place naturally in still air. Nevertheless, if the installations so allow, adapted cooling may be used in some cases either on account of the diameter of the parts (with parts of large size cooling that is too slow may lead to the onset of ferrite and/or pearlite in an excessive amount), or to obtain higher mechanical characteristics than those which would be obtained with natural cooling. Cooling with forced air may be sufficient to reach this objective. Attention must be paid however so that cooling is not rapid to the point of causing a massive onset of martensite as would happen with quenching.

In addition, tempering heat treatment at low temperature (200 to 350° C. for periods of 30 minutes to 4 hours), on the grades of the invention, allows a highly significant increase to be obtained in yield strength without increasing hardness and without reducing impact strength.

Depending on the parts concerned, several machining operations may be performed after forging or rolling and before or after tempering to obtain the precise dimensions and surface characteristics of the end part.

The mechanical characteristics being obtained by natural cooling, they are also able to be obtained from a hot rolled bar ready for use if it already has the desired metallurgical structure (essentially bainitic) which is described below. The composition of the steels used in the invention is such that the probability of obtaining the targeted structure naturally after mere cooling in air of the hot rolled bar under usual conditions is not negligible, if the dimensions of the bar lead to an adequate cooling rate.

The results are given below obtained with the steel compositions conforming to the requirements of the invention and those of reference compositions. These results were obtained on laboratory castings forged into 40 mm rounds or on industrial castings forged into rounds of equivalent diameter. To allow a significant comparison between results, the mechanical characteristics are evaluated after austenization at 1000° C. followed by natural cooling in still air or cooling under forced air. In addition, for reference, two bainitic grades were added which impart high mechanical characteristics to hot forgings and are already used on crankshafts, rails and other forged parts having high mechanical strength: specimen A (corresponding to EP-B-0 787 812) and B (corresponding to EP-A-1 426 453). The compositions of these samples are given in Table 1 together with their bainitic transformation onset temperature Bs calculated as previously on the basis of C, Mn and Cr contents.

TABLE 1 Compositions and Bs of the tested specimens. Bs Specimen C % Mn % Cr % Si % Ni % Mo % V % Others (° C.) A (Ref) 0.25 1.52 0.84 0.80 0.17 0.10 0.19 Ti, B 567 B (Ref) 0.29 1.13 1.15 0.80 0.20 0.12 0.18 Nb, Ti, B 570 C (Inventn) 0.18 1.62 1.51 0.18 0.17 0.19 0.09 530 D (Inventn) 0.16 1.68 1.54 0.94 0.06 0.20 0.09 Ti, B 528 E (Inventn) 0.21 1.78 1.38 1.06 0.10 0.19 0.09 Ti, B 517 F (Inventn) 0.18 1.60 1.54 0.86 0.96 0.19 0.20 Ti, B 530 G (Inventn) 0.18 1.53 1.45 0.97 0.10 0.17 0.09 Ti, B 542

The contents of Ti, Nb and B are typically 0.030%, 0.025% and 0.003% respectively when these elements are present.

Table 2 gives the mechanical characteristics measured on the products obtained from these specimens. It is to be pointed out here that the results obtained, in absolute value, are only be analysed in the precise context to which they refer. Differences are frequently observed in the mechanical properties obtained on forged or rolled parts of same composition but of different size, generally with an increase in mechanical characteristics for an equivalent diameter. The hierarchy between the examined grades will nevertheless remain identical for specimens all having the same dimensions which may be different from those of the examples cited herein. The indication “FA” after the specimen reference means that cooling in this case was forced air cooling.

TABLE 2 Mechanical characteristics of the specimens after austenization and cooling. Re Rm KCU Specimen Structure (MPa) (MPa) Re/Rm A (%) Z (%) (J · cm⁻²) A bainite 666 1114 0.60 19 39 39 B bainite 739 1226 0.60 18 41 27 A (FA) bainite 694 1119 0.62 14 30 32 C bainite 738 1185 0.62 15 53 50 D bainite 709 1173 0.60 14 44 44 D (FA) bainite 759 1203 0.63 15 57 69 E bainite 796 1303 0.61 15 39 47 E (FA) bainite + 10% 989 1344 0.74 12 46 58 martensite F bainite 745 1213 0.61 17 49 44 F (FA) bainite 774 1238 0.63 16 50 50 G bainite 769 1212 0.63 17 51

The mechanical characteristics of the steel specimens of the invention, C to G, therefore show a significant increase in mechanical strength compared with the medium carbon bainitic grades A and B whose carbon content comes under the medium-high carbon category. The yield strengths are 60 to 130 MPa higher and the mechanical strengths 70 to 190 MPa higher, other things being equal. They also allow an increase in impact strength of up to about 100% compared with medium-high carbon grades (C: 50 J/cm² compared with 39 J/cm² for A, 32 J/cm² for A (FA) and 27 J/cm² for B) again with other things being equal.

As indicated in Table 3, the structure is bainitic in all cases with the exception of the E (FA) casting cooled under forced air. This is supported by the Re/Rm ratio which works out at a value of about 0.6 typical of a bainitic structure, except in the case of E (FA) in which martensite is present and in which Re/Rm assumes a higher value.

The presence of martensite is not prohibitive in itself, insofar as the mechanical characteristics remain very high (in particular impact strength remains higher than 40 J/cm²). On the other hand, since the fraction of formed martensite is highly sensitive to the exact conditions of cooling, major dispersion of the mechanical characteristics can be expected in parts produced under industrial conditions for which the controlled cooling of the part cannot always be optimal. The objective must therefore be set of limiting the total presence of martensite, pro-eutectoid ferrite and pearlite to no more than 20%.

However, the major role must be stressed of the size of the parts when analysing mechanical characteristics: for example, while grade E cooled under forced air exhibits martensite on a round 40 mm in diameter, it was ascertained that conversely it allows a homogeneous bainitic structure to be guaranteed on diameters of 50 to 300 mm.

If a particularly high value of Re is sought, it is possible to apply tempering to the part at low temperature, before or after final machining. As shown in Table 3, such tempering allows a yield strength to be obtained that is up to 200 MPa higher than that obtained after normalization, whilst maintaining even increasing impact strength (up to +25%). Machining thereof will not be affected. It is also found that the results obtained vary little over a temperature range of 250-350° C. for tempering. Industrial treatment can therefore easily be carried out without the need for very precise control over tempering conditions.

TABLE 3 Mechanical characteristics obtained after tempering Re Rm KCU Specimn Treatment (MPa) (MPa) RE/Rm A (%) Z (%) (J · cm⁻²) F no temper 745 1213 0.61 17 49 44 temper 940 1184 0.79 12 59 59 300° C., 2 h temper 937 1165 0.80 15 60 57 350° C., 2 h C no temper 738 1185 0.62 15 53 50 temper 897 1156 0.78 15 58 250° C., 2 h temper 920 1144 0.80 14 60 300° C., 2 h temper 912 1138 0.80 14 56 350° C., 2 h 

1. A mechanical steel part in comprising in weight percentages: 0.05%≦C≦0.25%; 1.2%≦Mn≦2%; 1%≦Cr≦2.5%; wherein the contents of C, Mn and Cr are such that (830-270C %-90Mn %-70Cr %)≦560; 0<Si≦1.5%; 0<Ni≦1%; 0<Mo≦0.5%; 0<Cu≦1%; 0<V≦0.3%; 0<Al≦0.1%; 0<B≦0.005%; 0<Ti≦0.03%; 0<Nb≦0.06%; 0<S≦0.1%; 0<Ca≦0.006%; 0<Te≦0.03%; 0<Se≦0.05%; 0<Bi≦0.05%; 0<Pb≦0.1%; the remainder of the steel part being iron and impurities resulting from processing, and wherein the structure of the steel is bainitic and contains no more than a total of 20% of martensite and/or pro-eutectoid ferrite and/or pearlite.
 2. The part according to claim 1, wherein ≦0<Si≦0.3%.
 3. The part according to claim 1, wherein 0.8≦Si≦1.5%.
 4. The part according to claim 1, wherein Ni≦0.5%.
 5. The part according to claim 1, wherein 0.04%≦Mo≦0.5%.
 6. The part according to claim 1, wherein 0.05%≦V≦0.3%.
 7. The part according to claim 1, wherein 0.005%≦Al≦0.1%.
 8. The part according to claim 1, wherein 0.0005%≦B≦0.005% and 0<N≦0.0080% and Ti %≦3.5%.
 9. The part according to claim 1, wherein 0.005%≦Ti≦0.03%.
 10. The part according to claim 1, wherein 0.005%≦S≦0.1%.
 11. A process for manufacturing a steel part comprising the following steps: preparing a steel blank or bar whose composition conforms to claim 1; hot-working the blank or bar an austenitic phase by forging or rolling; cooling the hot-worked blank or bar at a rate that imparts a bainitic structure thereto, wherein the cooled, hot-worked blank or bar contains a total of no more than 20% martensite and/or pearlite and/or pro-eutectoid ferrite; and optionally carrying out one or more machining operation(s) to give the part its final dimensions and surface condition.
 12. The process according to claim 11 wherein, before or after the machining operation(s), tempering is performed over a temperature range of 200 to 350° C. for 30 minutes to 4 hours.
 13. The process according to claim 11, wherein the hot-worked blank or bar is cooled naturally in still air.
 14. The process according to claim 11, wherein the hot-worked blank or bar is cooled in forced air. 